It is known that the Mach number of an aircraft is defined as the ratio between the speed of the aircraft and the speed of sound in the flying conditions of the aircraft. The laws of thermodynamics make it possible to define a relationship between the Mach number of the airflow, the total pressure Pt and the static pressure Ps, which is formulated as follows:
  M  =                    2                  γ          -          1                    ⁡              [                                            (                              Pt                Ps                            )                                                      γ                -                1                            γ                                -          1                ]            
In this equation, γ represents the adiabatic index, which is equal to 1.4 for air. Using this equation, the measurements of the static pressure Ps and of the total pressure Pt are sufficient to calculate the Mach number M.
The static pressure Ps corresponds to the atmospheric pressure which depends on the altitude of the aircraft and on the temperature of the ambient medium. It can be measured using a probe placed on the aircraft.
Furthermore, the total pressure Pt corresponds to the sum of the static pressure and of the impact pressure due to the speed of the aircraft. The total pressure Pt is measured by Pitot probes.
In case of errors originating from the Pitot probes, the Mach number calculated using the abovementioned equation may therefore be wrong.
Patent CA2783222 discloses a method for measuring the Mach number using measurements of two static pressures at the level of the engine. These measurements make it possible to produce an estimation of a static pressure Ps and an estimation of a total pressure Pt. By introducing these estimated pressure values into the abovementioned equation, an estimation of the Mach number M is then obtained. However, the static pressure probe which makes it possible to produce such an estimation of a total pressure is not available on all types of aircraft and on all types of engines.